Multiplexing method in a multi carrier transmit diversity system

ABSTRACT

The invention relates to a method of multiplexing data words in a multicarrier transmit diversity system. The method comprises the step of generating a plurality of data blocks, each data block comprising data words and each data word containing data symbols derived from a data signal, the step of determining for one or more data blocks in dependence on at least one transmission constraint if the data words of said one or more data blocks are to be multiplexed in the time domain or in the frequency domain and the step of multiplexing the data words of the data blocks in accordance with the determination result.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] The present invention relates to the field of transmit antennadiversity and in particular to a method of multiplexing data words in amulti carrier transmit diversity system. The invention also relates to amultiplexer for multiplexing a sequence of data symbols and ademultiplexer for demultiplexing a multiplexed sequence of data symbols.

[0003] 2. Discussion of the Prior Art

[0004] Peak transmission rates in wireless communication systems havesteadily increased during the last years. However, peak transmissionrates are still limited for example due to path loss, limited spectrumavailability and fading.

[0005] Transmitter diversity is a highly effective technique forcombating fading in wireless communications systems. Several differenttransmit diversity schemes have been proposed. In Li, Y.; Chuang, J. C.;Sollenberger, N. R.: Transmitter diversity for OFDM systems and itsimpact on high-rate data wireless networks, IEEE Journal on Selec.Areas, Vol. 17, No. 7, July 1999 the transmit diversity schemes ofdelay, permutation and space-time coding are examplarily described.According to the delay approach, a signal is transmitted from a firsttransmitter antenna and signals transmitted from further transmitterantennas are delayed versions of the signal from the first transmitterantenna. In the permutation scheme, the modulated signal is transmittedfrom a first transmitter antenna and permutations of the modulatedsignal are transmitted from further transmitter antennas. By means ofspace-time coding a signal is encoded into several data words and eachdata word is transmitted from a different transmitter antenna. Duringtransmission the data words are spread (or multiplexed) in the timedomain by successively transmitting the data symbols of a data word overa single carrier frequency.

[0006] A further transmit diversity scheme for a multicarrier system isspace-frequency coding. By means of space-frequency coding a signal isencoded into several data words and each data word is spread (ormultiplexed) in the frequency domain by transmitting the data symbols ofeach data word on orthogonal frequencies, i.e. orthogonal subcarriers.An exemplary scheme for space-frequency coding is described in Mudulodu,S.; Paulraj, A.: A transmit diversity scheme for frequency selectivefading channels, Proc. Globecom, San Francisco, pp. 1089-1093, Nov.2000. According to the multicarrier system described in this paper, thedata words relating to an encoded signal are preferably multiplexed inthe time domain although orthogonal frequencies are available and thedata words could thus also be multiplexed in the frequency domain. Thisis due to the fact that if multiplexing in the frequency domain isutilized the employed frequencies, i.e. subcarriers, must see the samechannel, which may not always be possible in a frequency selectivefading channel. However, in case the subcarriers experience the samechannel, it is stated that either multiplexing in the time domain ormultiplexing in the frequency domain or a combination of the two may beused. By combining multiplexing in the time domain and in the frequencydomain the data symbols of a data word are simultaneously multiplexed inthe time domain and in the frequency domain. This means that the dataword is spread both across time and across frequencies.

[0007] Departing from the various transmit diversity schemes hithertoknown there is a need for a method of multiplexing data words in amulticarrier transmit diversity system which can easily be adapted tothe specifications of different wireless communications systems. Thereis also a need for a corresponding multiplexer and a demultiplexer.

BRIEF DESCRIPTION OF THE INVENTION

[0008] The existing need is satisfied by a method of multiplexing datawords in a multicarrier transmit diversity system which comprises thestep of generating a plurality of data blocks, each data blockcomprising data words and each data word containing data symbols derivedfrom a data signal, the step of determining for one or more of the datablocks in dependence on at least one transmission constraint if the datawords of said one of more data blocks are to be multiplexed in a timedomain or in a frequency domain and the step of multiplexing the datawords of the data blocks in accordance with the result of thedetermination.

[0009] The multiplexing method of the invention is not restricted to aspecific transmit diversity scheme as long as the utilized transitdiversity scheme enables to generate from a data signal a plurality ofdata blocks having the above structure. For example, the transmitdiversity schemes of block coding and of permutation allow to generatesuch data blocks. Preferably, the generated data blocks have thestructure of a matrix similar to a space-time block code (STBC) matrix.Also, it is not required that the transmit diversity scheme guaranteesfull transmit diversity. In other words: the invention does notnecessitate that each information symbol comprised within the datasignal is transmitted from each transmitter antenna. Nonetheless, apreferred embodiment of the invention comprises the feature of fulltransmit diversity.

[0010] Moreover, the invention is not restricted to any number oftransmit and receive antennas. Preferably, the number of data words perdata block equals the number of transmit antennas such that each dataword of a data block may be transmitted from an individual transmitterantenna. If more than one receive antenna is provided, the receivediversity scheme of maximum-ratio combining can be applied. However,other receive diversity schemes may be used as well.

[0011] According to the invention, it is decided on a data block levelhow the data words are to be multiplexed. The decision on the data blocklevel allows to change the multiplexing domain from one data block to asubsequent data block which is advantageous if one has to cope withspecific predefined or varying transmission constraints. Also, themultiplexing method according to the invention can be applied in variouswireless communication systems without major changes due to the specificmultiplexing flexibility gained by selecting the multiplexing domain onthe data block level. The multiplexing domain can be determined for eachdata block individually or simultaneously for a plurality of datablocks. For example, it can be decided for a sequence of data blocksthat all data words comprised within the sequence of data blocks are tobe multiplexed in either the time domain or in the frequency domain.

[0012] The multiplexing domain is determined by taking into account oneor more transmission constraints. For example, the transmissionconstraints may comprise one or more physical transmission constraintsor one or more data-related transmission constraints. It can alsocomprise both one or more physical transmission constraints and one ormore data-related transmission constraints. The physical transmissionconstraints relate to the physical transmission conditions and can bederived from physical transmission parameters like a channel coherencebandwidth or a coherence time. The data-related transmission constraintsrelate to system specific constraints regarding for example the employedmultiplexing scheme for the data words, the structure of the datasignal, the structure of the data blocks, the structure of the datawords or the structure of the data symbols.

[0013] The data symbols may be derived from the data signal in variousways dependent on the transmit diversity scheme which is used. If, forexample, the transmit diversity scheme of permutation is used, the datasymbols contained in the data words are permutations of informationsymbols comprised within the data signal. As a further example, if thetransmit diversity scheme of block coding is used, the data symbolscontained in the data words are obtained from the information symbolscomprised within the data signal by means of permutation and basicarithmetic operations, such as negation and complex conjugation.

[0014] The data signal from which the one or more data blocks aregenerated can have any format. According to a preferred embodiment, thedata signal has the format of a sequence of discrete informationsymbols. For example, the data signal may have the structure of vectors,each vector comprising a predefined number of information symbols. Thenature of the information symbols may depend on the specific wirelesscommunication system in which the multiplexing method according to theinvention is used. Many wireless communication systems employ differenttypes of information symbols for different purposes. For example, somewireless communication systems use data signals which comprise apreamble, one or more user data sections or both a preamble and one ormore user data sections. Usually, the preamble has a predefinedstructure and is utilized for purposes like channel estimation,frequency synchronization and timing synchronization.

[0015] In the following, several exemplary data-related transmissionconstraints are described in more detail. According to a firstembodiment, the data-related transmission constraint is a predefinednumber N of data symbols to be comprised within each data word which isto be multiplexed in the time domain.

[0016] Usually, the number N of data symbols to be comprised within eachdata word cannot arbitrarily be chosen because it may depend for exampleon a code rate, on the condition that the data blocks have to beorthogonal matrices or on the availability of memory resources withinthe multicarrier transmit diversity system.

[0017] When the data words of a specific data block are to bemultiplexed in the time domain, the number N of data symbols to becomprised within each data word may represent the number of time slotsrequired for the transmission of a single data word over a singlesubcarrier. On the other hand, when the data words of a specific datablock are to be multiplexed in the frequency domain, the number N ofdata symbols to be comprised within each data word stands for the numberof subcarriers required to transmit a single data word during a singletime slot.

[0018] Preferably, all data words of an individual data block comprisethe same number of data symbols. If the data signal has such a structurethat the number of data symbols comprised within each data word of aspecific data block equals the predefined number N of data symbols, thedata words of this data block may be multiplexed in the time domain.Otherwise, i.e. if the data signal has such a structure that the numberof data symbols comprised within each data word of a specific data blockdoes not equal the predefined number N of data symbols, the data wordsof this data block may be multiplexed in the frequency domain. Such adistinction will become necessary if the data signal or a portionthereof has a predefined length because the predefined length may implythat the total number N_(D) of data symbols which corresponds to thepredefined length of the data signal or a portion thereof is not aninteger multiple of the predefined number N of data symbols which shouldbe comprised within a data word to be multiplexed in the time domain. Insuch a situation integer multiples of the predefined number N of datasymbols are arranged in data blocks of data words which are multiplexedin a time domain and a remainder N_(R)=mod(N_(D)/N) of data symbols isarranged in a data block with data words which are multiplexed in thefrequency domain.

[0019] Thus, by combining multiplexing in the time domain and in thefrequency domain, data symbol fitting problems resulting from thepredefined number N of data symbols to be comprised within each dataword which is to be multiplexed in the time domain can be solved. Suchdata symbol fitting problems may for example become relevant when thedata signal or a portion of the data signal has a predefined lengthbecause the wireless transmission system necessitates that the preambleportion or the user data portion of a data signal comprises a certainnumber of information symbols. Thus the data words of all data blocksexcept for the last data block are multiplexed in the time domain andthe data words of the last data block are either multiplexed in the timedomain or in the frequency domain depending on whether or not the datawords of the last data block contain a number of data symbols whichequals the predefined number N of data symbols.

[0020] So far the data-related transmission constraint of a predefinednumber N of data symbols to be comprised within each data word has beenillustrated. According to a second embodiment, the data signal maycomprise one or more periodic structures and the data relatedtransmission constraint may be a preservation of the periodic structuressuch that the periodic structures are still periodic on a receiver side.The one or more periodic structures may be comprised within a preambleof the data signal, for example in the form of two or more identicalpreamble information symbols. Periodic structures are advantageousbecause they allow the use of synchronization algorithms withcomparatively low complexity.

[0021] In case of multiplexing data symbols relating to periodicstructures in the time domain the periodicity of the periodic structuresmay get lost. Therefore, at least the data words of data blocks whichrelate to the periodic structures or parts of periodic structures aremultiplexed in the frequency domain. By multiplexing the data words ofthese data blocks in the frequency domain it can be ensured that theperiodicity of the periodic structures is maintained.

[0022] When the data words of data blocks generated from periodicstructures or portions thereof are multiplexed in the frequency domain,the data words of data blocks generated from the remaining data signalare preferably multiplexed in the time domain. If, for example, the datawords of data blocks generated from a preamble comprising periodicstructures are multiplexed in the frequency domain, the data words ofdata blocks generated from a corresponding user data section may bemultiplexed at least partly in the time domain.

[0023] Instead of data-related transmission constraints or in additionto data-related transmission constraints physical transmissionconstraints can be taken into account when deciding if the data words ofone or more specific data blocks are to be multiplexed in the timedomain or in the frequency domain. According to a preferred embodiment,the decision is made based on simultaneously evaluating a combination ofone or more data-related transmission constraints and one or morephysical transmission constraints.

[0024] The physical transmission constraints may be determined based onat least one of a channel coherence bandwidth

B _(C)≈1/τ_(rms)  (1)

[0025] and a coherence time

t _(c)≈1/(2·f _(D))  (2)

[0026] wherein f_(D) is the doppler frequency and τ_(rms) is the rootmean square of the delay spread of the channel impulse response.

[0027] Many transmit diversity schemes require constant or at leastapproximately constant channel parameters during transmission of onedata word. If the data words are to be multiplexed in the frequencydomain, a comparatively large coherence band width is required. Thismeans that the relation

B _(C) >>N/T  (3)

[0028] has to be fulfilled at least approximately, wherein N is thenumber of data symbols per data word and T is the duration of one of thedata symbols, i.e. the duration of one time slot. A comparatively largecoherence bandwidth requires that the channel parameters of N adjacentsubcarriers have to be almost constant.

[0029] On the other hand, if the data words are to be multiplexed in thetime domain, a comparatively large coherence time is required. Thismeans that the relation

t _(c) >>T·N  (4)

[0030] has to be fulfilled at least approximately. In other words: Nsubsequent data symbols have to have nearly constant channel parameters,i.e. the channel parameters for a single subcarrier have to remainconstant for a period of N·T.

[0031] The physical transmission constraint may be determined byassessing if one or both of the relations (3) and (4) are fulfilled.Dependent on which of the two relations (3) and (4) is fulfilled best itis decided that the data words of the data blocks are to be multiplexedeither in the time domain or in the frequency domain as a general rule.Deviations from this general rule may become necessary due todata-related transmission constraints. For example, the data symbolfitting problem or the problem encountered with periodic structures maynecessitate that although multiplexing in the time domain is generallyto be preferred, the data words of at least some data blocks have to bemultiplexed in the frequency domain. As a further example, changingtransmission conditions may necessitate that the data words of some datablocks have to be multiplexed in the time domain and the data words ofother data blocks have to be multiplexed in the frequency domain. As athird example, the data words of data blocks generated from a preamblemay be multiplexed in the time domain and the data words of data blocksgenerated from a user data section may be multiplexed in the frequencydomain. Such a combination has the advantage that the above-mentioneddata symbol fitting problem, which usually is most relevant for the userdata section, can be avoided while the multiplexing in the time domainof the data words of data blocks generated from the preamble allows agood channel estimation.

[0032] It was mentioned above that in order to achieve full diversityeach information symbol has to be transmitted from each transmitterantenna. A further requirement of full transmit diversity is that theantenna signals are orthogonal to each other. This means that the datasymbols have to be modulated onto subcarriers which are orthogonal toeach other. However, the invention can also be practiced in case thesubcarriers are not orthogonal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Further advantages of the invention will become apparent byreference to the following description of a preferred embodiment of theinvention in the light of the accompanying drawings, in which:

[0034]FIG. 1 shows a data signal in the form of a physical burst to beprocessed in accordance with the invention;

[0035]FIG. 2 is a block diagram of a transceiver for wirelesscommunication adapted to multiplex data words in accordance with theinvention;

[0036]FIG. 3 shows several modulation schemes defined in the HIPERLAN/2standard;

[0037]FIG. 4 shows the block code encoder of the transceiver depicted inFIG. 2;

[0038]FIG. 5 shows the configuration of a transmit antenna diversityscheme;

[0039]FIG. 6 is a schematic diagram of multiplexing data words in thetime domain in accordance with the invention; and

[0040]FIG. 7 is a schematic diagram of multiplexing data words in thefrequency domain in accordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0041] Although the present invention can be used in any multicarriertransmit diversity system which employs a transmit diversity schemeallowing to generate data blocks having a structure as described above,the following description of preferred embodiments is examplarily setforth with respect to a multicarrier system which employs orthogonalfrequency division multiplexing (OFDM) and which utilizes block codingfor generating data blocks from a data signal.

[0042] The exemplary multicarrier system described below is derived fromthe European wireless local area network (WLAN) standard highperformance radio local area network type 2 (HIPERLAN/2). HIPERLAN/2systems are intended to be operated in the 5 GHz frequency band. Asystem overview of HIPERLAN/2 is given in ETSI TR 101 683, BroadbandRadio Access Networks (BRAN); HIPERLAN Type 2; System Overview, V1.1.1(2000-02) and the physical layer of HIPERLAN/2 is described in ETSI TS101 475; Broadband Radio Access networks (BRAN); HIPERLAN Type 2;Physical (PHY) Layer, V1.1.1 (2000-04). The multicarrier scheme of OFDM,which is specified in the HIPERLAN/2 standard, is very robust infrequency selective environments.

[0043] Up to now, the HIPERLAN/2 system and many other wirelesscommunications systems do not support transmit diversity in spite of thefact that transmit diversity would improve the transmission performanceand reduce negative effects of fast fading like Rayleigh fading.However, applying standard transmit diversity schemes to multicarriercommunications systems may lead to various problems which arehereinafter examplarily described with respect to the HIPERLAN/2 system.

[0044] In FIG. 1 a typical physical burst of HIPERLAN/2 is illustrated.The physical burst comprises a preamble consisting of preamble symbolsand a user data section consisting of user data symbols. In HIPERLAN/2five different physical bursts are specified and each kind of physicalburst has a unique preamble. However, the last three preamble symbolsconstitute a periodic structure which is identical for all preambletypes. This periodic structure consists of a short OFDM symbol C32 of 32samples followed by two identical regular OFDM symbols C64 of 64samples. The short OFDM symbol C32 is a cyclic prefix which is arepetition of the second half of one of the C64 OFDM symbols. Theso-called C-preamble depicted in FIG. 1 is used in HIPERLAN/2 forchannel estimation, frequency synchronization and timingsynchronization. The periodic structure within the C-preamble isnecessary in order to allow the use of synchronization algorithms withcomparatively low complexity.

[0045] The user data section of the physical burst depicted in FIG. 1comprises a variable number N_(SYM) of OFDM symbols required to transmita specific protocol data unit (PDU) train. Each OFDM symbol of the userdata section consists of a cyclic prefix and a useful data part. Thecyclic prefix consists of a cyclic continuation of the useful data partand is inserted before it. Thus, the cyclic prefix is a copy of the lastsamples of the useful data part. The length of the useful data part isequal to 64 samples and has a duration of 3.2 μs. The cyclic prefix hasa length of either 16 (mandatory) or 8 (optional) samples and a durationof 0.8 μs or 0.4 μs, respectively. Altogether, a OFDM symbols thus has alength of either 80 or 72 samples corresponding to a symbol duration of4.0 μs or 3.6 μs, respectively. An OFDM symbol therefore has anextension in the time domain. A OFDM symbol further has an extension inthe frequency domain. According to HIPERLAN/2, a OFDM symbol extendsover 52 subcarriers. 48 subcarriers are reserved for complex valuedsubcarrier modulation symbols and 4 subcarriers are reserved for pilots.

[0046] From the above it becomes clear that the HIPERLAN/2 physicalburst depicted in FIG. 1 has a predefined length both in a timedirection and in a frequency direction. Moreover, the physical burst ofFIG. 1 comprises a periodic structure. It are among others thesefeatures of the physical burst of FIG. 1 which may lead to problems whenthe HIPERLAN/2 system or a similar wireless communication system has tobe adapted to transmit diversity.

[0047] For typical HIPERLAN/2 scenarios the above relation (4) isusually fulfilled because the doppler frequency f_(D) is comparativelylow. However, especially in outdoor environments, relatively large delayspreads can occur. Consequently, relation (3) cannot always befulfilled. Therefore, a transmit diversity scheme like STBC multiplexingin the time domain should generally be a preferred transmit diversityscheme for a HIPERLAN/2 scenario from the point of view that the channelover one space-time data word should be as constant as possible.However, severe problems arise when STBC is applied to physical burstshaving the structure depicted in FIG. 1 or a similar structure.

[0048] Both the physical burst and the OFDM symbols comprised thereinhave predefined dimensions in the time domain and in the frequencydomain. Concurrently, STBC requires that each STBC data word has apredetermined length N. Thus, data unit fitting problems arise if thedimension of e.g. an OFDM symbol of the preamble or of the user datasection cannot be mapped on an integer multiple of the length of oneSTBC data word. Moreover, when applying STBC to the periodic C-preambledepicted in FIG. 1, the periodicity of the C-preamble gets lost. This isdue to the fact that the one or more STBC data words relating to thesecond C64 OFDM symbol will no longer be equal to the one or more STBCdata words relating to the first C64 OFDM symbol. The loss ofperiodicity, however, leads to the problem that the symbolsynchronization algorithms which make use of a periodic structure withinthe preamble can no longer be employed. Also, the C32 OFDM symbol cannotserve any longer as a guard interval separating the OFDM symbols withinthe preamble. The reason therefore is that in case of multipathpropagation the first C64 OFDM symbol interferes with the second C64OFDM symbol which is no longer equal to the first C64 OFDM symbol.

[0049] The above problems and further problems not explicitly discussedabove do not occur when the data words are multiplexed in accordancewith the invention. In FIG. 2, the physical layer of a transceiver 10which is adapted to implement the method according to the invention isillustrated. The transceiver 10 comprises a scrambler 12, an FEC codingunit 14, an interleaving unit 16, a mapping unit 18, an OFDM unit 20, aburst forming unit 22, a block code encoder 24, a multiplexer 26, aradio transmitter 30 and a control unit 32. The block code encoder 24and the multiplexer 26 together form an encoder/multiplexer unit 28.

[0050] The transceiver 10 depicted in FIG. 1 receives as input signal aPDU train from a data link control (DLC). Each PDU train consists ofinformation bits which are to be framed into a physical burst, i.e. asequence of OFDM symbols to be encoded, multiplexed and transmitted.

[0051] Upon receipt of a PDU train the transmission bit rate within thetransceiver 10 is configured by choosing an appropriate physical modebased on a link adaption mechanism. A physical mode is characterized bya specific modulation scheme and a specific code rate. In the HIPERLAN/2standard several different coherent modulation schemes like BPSK, QPSK,16-QAM and optional 64-QAM are specified. Also, for forward errorcontrol, convolutional codes with code rates of 1/2, 9/16 and 3/4 arespecified which are obtained by puncturing of a convolutional mothercode of rate 1/2. The possible resulting physical modes are depicted inFIG. 3. The data rate ranging from 6 to 54 Mbit/s can be varied by usingvarious signal alphabets for modulating the OFDM subcarriers and byapplying different puncturing patterns to a mother convolutional code.

[0052] Once an appropriate physical mode has been chosen, the N_(BPDU)information bits contained within the PDU train are scrambled with thelength-127 scrambler 12. The scrambled bits are then output to the FECcoding unit 14 which encodes the N_(BPDU) scrambled PDU bits accordingto the previously set forward error correction.

[0053] The encoded bits output by the FEC coding unit 14 are input intothe interleaving unit 16 which interleaves the encoded bits by using theappropriate interleaving scheme for the selected physical mode. Theinterleaved bits are input into the mapping unit 18 where sub-carriermodulation by mapping the interleaved bits into modulation constellationpoints in accordance with the chosen physical mode is performed. Asmentioned above, the OFDM subcarriers are modulated by using BPSK, QPSK,16-QAM or 64-QAM modulation depending on the physical mode selected fordata transmission.

[0054] The mapping unit 18 outputs a stream of complex valued subcarriermodulation symbols which are divided in the OFDM unit in groups of 48complex numbers. In the OFDM unit a complex base band signal is producedby OFDM modulation as described in ETSI TS 101 475, Broadband RadioAccess Networks (BRAN); HIPERLAN Type 2; Physical (PHY) Layer, V1.1.1(2000-04).

[0055] The complex base band OFDM symbols generated within the OFDM unit20, where pilot subcarriers are inserted, are input into the physicalburst unit 22, where an appropriate preamble is appended to the PDUtrain and the physical burst is built. The physical burst produced bythe physical burst unit 22 has a format as depicted in FIG. 1. Thephysical burst unit 22 thus outputs a sequence of complex base band OFDMsymbols in the form of the physical burst to the block code encoder 24.

[0056] The function of the block code encoder 24 is now generallydescribed with reference to FIG. 4. In general, the block code encoder24 receives an input signal in the form of a sequence of vectors X=[X₁X₂. . . X_(K)]^(T) of the length K. The block code encoder 24 encodes eachvector X and outputs for each vector X a data block comprising aplurality of signal vectors C⁽¹⁾, C⁽²⁾ . . . , C^((M)) as depicted inFIG. 4. Each signal vector C⁽¹⁾,C⁽²⁾ . . . , C^((M)) corresponds to asingle data word. Thus, the data block generated from the vector Xcomprises M data words wherein M is the number of transmitter antennas.

[0057] Each data word C^((i)) with i=1 . . . M comprises N data symbols,i.e. each data word C^((i)) has a length of N. The value of N cannot befreely chosen since the matrix C spanned by the data words C^((i)) hasto be orthogonal in this embodiment. Several examples for data blocks inthe form of orthogonal code matrices C are described in U.S. Pat. No.6,088,408. In the block coding approach described in the presentembodiment all data symbols c_(j) ^(i) of the code matrix C are derivedfrom the components of the input vector X and are simple linearfunctions thereof or of its complex conjugate.

[0058] If a receive signal vector Y at one receive antenna is denoted byY=[Y₁Y₂ . . . Y_(N)]^(T), the relationship between Y and the code matrixC is as follows: $\begin{matrix}{{\begin{bmatrix}Y_{2} \\\cdots \\Y_{N}\end{bmatrix} = {\begin{bmatrix}c_{2}^{\text{?}} & \cdots & \quad & c_{2}^{\text{?}} \\\cdots & \quad & \cdots & \cdots \\c_{N}^{(1)} & c_{N}^{(2)} & \cdots & c_{N}^{(M)}\end{bmatrix} \cdot \begin{bmatrix}n^{\text{?}} \\\cdots \\h^{(M)}\end{bmatrix}}}{\text{?}\text{indicates text missing or illegible when filed}}} & (5)\end{matrix}$

[0059] where h^((i)) represents the channel coefficient of the channelfrom the i-th transmit antenna to the receive antenna. A generalizationto more receive antennas is straightforward.

[0060] In the following examples of possible block code matrices for twoand three transmitter antennas, respectively, are discussed in moredetail. The configuration of a wireless communication system with twotransmit antennas and one receive antenna is depicted in FIG. 5. For twotransmit antennas one possible block code matrix C with a code rate R=1is: $\begin{matrix}{C = \begin{bmatrix}X_{1} & X_{2} \\{- X_{2}^{*}} & X_{2}^{*}\end{bmatrix}} & (6)\end{matrix}$

[0061] For three transmit antennas one possible block code matrix C witha code rate R=0.5 is: $\begin{matrix}{{c = \begin{bmatrix}\text{?} & \text{?} & \text{?} \\{- X_{4}} & {- X_{3}} & X_{2} \\X_{1}^{*} & X_{2}^{*} & X_{3}^{*} \\{- X_{2}^{*}} & X_{1}^{*} & {- X_{4}^{*}} \\{- X_{3}^{*}} & X_{4}^{*} & X_{1}^{*} \\{- X_{4}^{*}} & {- X_{3}^{*}} & X_{2}^{*}\end{bmatrix}}{\text{?}\text{indicates text missing or illegible when filed}}} & (7)\end{matrix}$

[0062] The code rate R is defined as the ratio of the length K of theinput vector X and the length N of each code word C^((i)):

R=K/N  (8)

[0063] As can be seen from FIG. 4, the block code encoder 24 outputs foreach data signal in the form of a vector X a data block in the form of amatrix C. The data block output by the block code encoder 24 is inputinto the multiplexer 26 which multiplexes the data words (vectorsC^((i))) of each data block in accordance with an externally providedcontrol signal either in the time domain or in the frequency domain. Thecontrol signal is generated by the control unit 32 based on anassessment of the transmission constraints. The assessment of thetransmission constraints and the controlling of the multiplexer 26 bymeans of the control unit 32 will be described later in more detail.

[0064] In the multicarrier scheme OFDM, the output of the block codeencoder 24 is modulated onto subcarriers which are orthogonal to eachother. There exist essentially two possibilities to multiplex a datablock comprising individual data words in an OFDM system. According to afirst possibility depicted in FIG. 6, the data words of a specific datablock are extended in the time direction (STBC). In other words: Thedata words are multiplexed in the time domain. According to a secondpossibility, the data words of a data block are extended in thefrequency direction as depicted in FIG. 7. This means that the datawords are multiplexed in the frequency domain. Multiplexing the datawords of a data block in the form of a code matrix in the frequencydomain will in the following be referred to as space-frequency blockcoding (SFBC).

[0065] As can be seen from FIGS. 6 and 7, the individual data words of adata block are transmitted from different transmit antennas. Accordingto the multiplexing scheme of FIG. 6, an individual data block istransmitted on an individual subcarrier over a time interval of N·T,wherein N is the number of data symbols per data word and T is theduration of one of the data symbols. According to the multiplexingscheme of FIG. 7, an individual data block is spread over N subcarriersand is transmitted during a time interval of T. It can clearly be seenthat the multiplexing scheme of FIG. 6 can generally be employed whenthe relation (4) is fulfilled and the multiplexing scheme of FIG. 7 cangenerally be employed when the relation (3) is satisfied.

[0066] The encoded and multiplexed output signal of theencoder/multiplexer unit 28 is input into the radio transmitter 30. Theradio transmitter 30 performs radio transmission over a plurality oftransmit antennas by modulating a radio frequency carrier with theoutput signal of the encoder/multiplexer unit 28. The transceiver 10 ofFIG. 2 further comprises a receiver stage not depicted in FIG. 2. Thereceiver stage has a physical layer with components for performing theinverse operations of the components depicted in FIG. 2. For example,the receiver stage comprises a descrambler, a FEC decoding unit, ademultiplexer/decoder unit with a demultiplexer and a block codedecoder, etc.

[0067] Now, the control of the multiplexer 26 will be described in moredetail with reference to both physical and data-related transmissionconstraints that may occur if physical bursts as the one depicted inFIG. 1 are employed. In accordance with typical HIPERLAN/2 scenarios, itis supposed that relation (4) is fulfilled and that it cannot always beguaranteed that relation (3) is fulfilled. This corresponds to therealistic situation that the basic performance of STBC transmission isbetter than the basic performance of SFBC transmission. Basicperformance here means that only physical transmission constraints aretaken into account. In such a case the control unit 32 may decide thatthe data blocks have to be multiplexed in the time domain. However, ifthe physical transmission parameters change, there might occur the casewhere relation (4) is no longer fulfilled whereas relation (3) isfulfilled at least approximately. In this case the control unit 32 willdecide that the data words of the data blocks are no longer multiplexedin the time domain. Instead, the control unit 32 controls themultiplexer 26 such that the data words of the data blocks aremultiplexed in the frequency domain.

[0068] So far only physical transmission constraints have beenconsidered. Should data-related transmission constraints also be ofimportance, the control unit 32 controls the multiplexer 26 byadditionally taking into account data-related transmission constraints.

[0069] It has been mentioned above that the transmission constraintswhich have to be considered in context with the physical burst depictedin FIG. 1 are the preservation of a periodic structure in the C-preambleand the provision of a predefined number N of data symbols in each dataword which is to be multiplexed in the time domain. These twodata-related transmission constraints can occur in several combinations.

[0070] According to a first scenario, the data signal has the structureof the physical burst depicted in FIG. 1 and comprises a user datasection and a preamble with a periodic structure. It is further supposedthat the data-related transmission constraint of preserving the periodicstructure has to be taken into account while no data symbol fittingproblem occurs with respect to the user data section. In such a case thedata words of data blocks relating to the preamble are multiplexed inaccordance with SFBC in the frequency domain and the data words of datablocks relating to the user data section are multiplexed in accordancewith STBC in the time domain. By multiplexing the data words derivedfrom the preamble in the frequency domain a preservation of the order ofthe C32 OFDM symbols and the two C64 OFDM symbols can be achieved.

[0071] According to a second scenario derived from the physical burstdepicted in FIG. 1, the periodic structure within the preamble has to bepreserved and additionally the data symbol fitting problem has to betaken into account with respect to the user data section. Like in thefirst scenario, the data words of data blocks derived from the preambleare multiplexed in accordance with SFBC in the frequency domain. Due tothe data symbol fitting problem the data words of the last data blockrelating to the user data structure contains less than the predefinednumber N of data symbols contained in data words of the previous datablocks. Therefore, only the data words (containing the predefined numberN of data symbols) of the previous data blocks are multiplexed inaccordance with STBC in the time domain. The data words of the last datablock contain N_(R)=mod(N_(D)/N) data symbols and are multiplexed inaccordance with SFBC in the frequency domain, wherein N_(D) is the totalnumber of data symbols to be transmitted over one transmit antenna.

[0072] According to a third scenario, the data-related transmissionconstraint of the preservation of a periodic structure within thepreamble is not relevant but the data symbol fitting problem is relevantwith respect to the user data section. In this case the data words ofdata blocks relating to the preamble are multiplexed in accordance withSTBC in the time domain and the data words of data blocks relating tothe user data section are multiplexed as described above for the secondscenario. In other words: The data words of the last data block have alength of N_(R) data symbols and the data words of the previous datablocks have the predefined length of N data symbols.

[0073] According to a fourth scenario, the data-related transmissionconstraint of preserving a periodic structure has not to be taken intoaccount and the physical transmission constraint of B_(C)>>N/T is atleast approximately fulfilled. In this case the data words of datablocks relating to the preamble are multiplexed in accordance with STBCin the time domain and the data words of data blocks relating to theuser data section are multiplexed in accordance with SFBC in thefrequency domain. By using STBC for the preamble a good channelestimation can be performed. Due to the use of STBC for the preamble theslightly worse performance of SFBC can be compensated by means ofreceiver algorithms for interference suppression based on the goodchannel estimation. Using STBC for the preamble and SFBC for the userdata section has the advantage that data symbol fitting problems withrespect to the user data section do not appear.

[0074] Additional scenarios based on further combinations ofdata-related and physical transmission constraints can easily berealized in accordance with the invention. Also, the invention caneasily be applied to data signals having a structure different from thestructure of the physical burst depicted in FIG. 1. Although theinvention is preferably practiced with the transmit diversity scheme ofa combination of STBC and SFBC, other transmit diversity schemes can beused as well.

1. A method of multiplexing data words in a multicarrier transmitdiversity system, comprising: a) generating a plurality of data blocks(C), each data block (C) comprising data words (C^((i))) and each dataword (C^((i))) containing data symbols (C_(j) ^((i))) derived from adata signal; b) determining for one or more data blocks (C) independence on at least one transmission constraint if the data words(C^((i))) of said one or more data blocks (C) are to be multiplexed inthe time domain or in the frequency domain; and c) multiplexing the datawords (C^((i))) of the data blocks (C) in accordance with thedetermination in step b).
 2. The method according to claim 1, whereinthe data signal comprises at least one of a preamble and a user datasection.
 3. The method according to claim 1 or 2, wherein the at leastone transmission constraint comprises a data-related transmissionconstraint.
 4. The method according to claim 3, wherein the data-relatedtransmission constraint is a pre-defined number (N) of data symbols(c_(j) ^((i))) to be comprised within each data word (C^((i))) which isto be multiplexed in the time domain.
 5. The method according to claim4, wherein the data words (C^((i))) containing the predefined number (N)of data symbols (c_(j) ^((i))) are multiplexed in the time domain andthe data words (C^((i))) containing more or less data symbols (c_(j)^((i))) are multiplexed in the frequency domain.
 6. The method accordingto claim 4 or 5, wherein the data signal or a portion thereof has apredefined length and wherein integer multiples of the predefined numberof data symbols (c_(j) ^((i))) are arranged in data blocks (C) with datawords (C^((i))) which are multiplexed in the time domain and a remainderof data symbols (c_(j) ^((i))) is arranged in data blocks (C) with datawords (C^((i))) which are multiplexed in the frequency domain.
 7. Themethod according to claim 6, wherein the user data section of the datasignal has the predefined length.
 8. The method according to claim 7,wherein the data words (C^((i))) of data blocks (C) relating to thepreamble are either multiplexed completely in the frequency domain orcompletely in the time domain dependent on the transmission constraint.9. The method according to one of claims 1 to 8, wherein the data signalcomprises one or more periodic structures (C32, C64).
 10. The methodaccording to claim 9, wherein the one or more periodic structures (C32,C64) are contained within the preamble.
 11. The method according toclaim 9 or 10, wherein the data-related transmission constraint is apreservation of the one or more periodic structures (C32, C64).
 12. Themethod according to one of claims 9 to 11, wherein at least the datawords (C^((i))) of data blocks (C) relating to the periodic structures(C32, C64) are multiplexed in the frequency domain.
 13. The methodaccording to claim 12, wherein the data words (C^((i))) of data blocks(C) relating to the user data section are multiplexed in the timedomain.
 14. The method according to one of claims 1 to 13, wherein theat least one transmission constraint comprises a physical transmissionconstraint.
 15. The method according to claim 14, wherein the physicaltransmission constraint is determined based on at least one of acoherence bandwidth and a coherence time.
 16. The method according toclaim 15, wherein the physical transmission constraint is determined byassessing if the relationship B_(C)>>N/T is fulfilled, wherein B_(C) isthe coherence bandwidth, N is the number of data symbols (c_(j) ^((i)))per data word (C^((i))) and T is the duration of one of the data symbols(c_(j) ^((i))).
 17. The method according to claim 15 or 16, wherein thephysical transmission constraint is determined by assessing if therelationship t_(c)>>N·T is fulfilled, wherein t_(c) is the coherencetime, N is the number of data symbols (c_(j) ^((i))) per data word(C^((i))) and T is the duration of one of the data symbols (c_(j)^((i))).
 18. The method according to claim 16 or 17, wherein, when thephysical transmission constraint B_(C)>>N/T is at least approximatelyfulfilled, the data words (C^((i))) of data blocks (C) relating to thepreamble are multiplexed in the time domain and the data words (C^((i)))of data blocks (C) relating to the user data sequence are multiplexed inthe frequency domain.
 19. The method according to one of claims 1 to 18,wherein the data blocks (C) are obtained from the data signal by meansof block coding or by means of permutation.
 20. The method according toone of claims 1 to 19, wherein the data symbols (c_(j) ^((i))) aremodulated onto subcarriers which are orthogonal to each other.
 21. Amultiplexer (26) adapted to multiplex data words in accordance with themethod according to one of claims 1 to
 20. 22. A demultiplexer adaptedto demultiplex data words multiplexed by the multiplexer of claim 21.23. A transceiver for wireless communication, comprising at least one ofa multiplexer according to claim 21 and a demultiplexer according toclaim 22.